Earth can be either a proper noun or a common noun. In English, proper nouns (nouns which signify a particular person, place, or thing) are capitalized. Following this rule, when Earth is discussed as a specific planet or celestial body, it is capitalized: It takes six to eight months to travel from Earth to Mars.

Should Earth be capitalized?

We usually lowercase sun, moon, and earth, but, following The Chicago Manual of Style, when the does not precede the name of the planet, when earth is not part of an idiomatic expression, or when other planets are mentioned, we capitalize earth: The earth revolves around the sun. The astronauts landed on the moon.

Is it Earth or the Earth?

When used as a common noun, it's frequently lowercase, and used with the article ("the"). When it's a proper noun -- like a person's name, "Bob", or even "God" vs "the god" -- then it's capitalized ("Earth"), and often without the article ("the").